The endocrinology of the reproductive system is based on systemic endocrine signalling and on the less readily intercepted signals between cells (paracrine) and within cells (autocrine and intracrine) which control local phenomena. The mechanism controlling parturition in women is a unique and local phenomenon. The most appropriate model for study, is the fetal/maternal interface, provided by the cells and extracellular matrix of the fetal amnion and chorion and the maternal decidua of the human fetal membranes. These are readily accessible, but only at a limited number of time points, for observation in vivo and experimentation in vitro. Hence, in normal tissues obtained at term, an autocrine/paracrine system has already been defined based on human relaxins and their control of specific metalloproteinases, an activator and inhibitor. The control of collagen breakdown and the finding of a novel elastic system in the amnion-chorion now explains fetal membrane accommodation and elasticity for the first time. Preterm premature rupture of the fetal membranes in the absence of infection resulting in preterm birth is a significant problem in perinatal medicine and can now be studied here by (1) seeking evidence in vivo for the excessive production of relaxins an-or expression of the relaxin receptor in women delivering preterm at 20-36 weeks gestation. (2) demonstrating a relationship between the increased relaxin/relaxin receptor expression and increased collagenolysis in vivo and in vitro, thereby linking relaxins and the preterm premature rupture of the fetal membranes. The combined results will show that the relaxins are involved in the preterm premature rupture of the fetal membranes and preterm birth: a significant problem in perinatal medicine in this country today.